CMD + L for lock screen

I haven't found one. What i do use is "hot corners" and set my screensaver to start when i drag my mouse to the lower right corner.

This should do it

#!/bin/bash

sudo -u $3 defaults write -g NSUserKeyEquivalents -dict-add "Lock Screen" -string "@l"

Though I found I needed to reboot for it to work globally after applying
Until I rebooted it would only work in some applications

Command + L is a very common key shortcut to go to a browser's URL Bar. Why not just teach your users about the builtin key command - Control+Command+Q?

If you are not paying attention and hit Option+Command+Q you just quit the current app, so I can see why you might want to map a different series of keys. Command+L is probably not the best choice to lock the screen because of what TimArnold stated above.

I'd like to echo @TimArnold 's comment.

Unless there's an accessibility issue, I'd err on the side of keeping the defaults so that users aren't confused when they're outside of the managed bubble.